1. Field of the Invention
The present invention generally relates to direct part markings on workpieces and electro-optically reading such markings by image capture and, more particularly, to improvements in aiming, ranging and illuminating such markings, as well as calibrating imaging readers during manufacture, and self-calibrating imaging readers during reading of such markings or other indicia.
2. Description of the Related Art
Direct part marking (DPM) allows workpieces to be directly marked, identified and traced to their origin, and its use is growing in the automotive, aerospace, electronics, medical equipment, tooling, and metalworking industries, among many others. Despite the ability to control very tight specifications on element size, width, spacing and so on, the lack of sharp contrast of machine-readable optical DPM codes directly marked on metal, plastic, leather, glass, etc., workpieces prevents traditional moving laser beam readers from electro-optically reading the DPM codes reliably. These moving beam readers emit a laser beam which reflects off the metal or glass workpieces as bright light.
To counter a variety of problems, such as lack of contrast, difficulty of maintaining precise element specifications, limited available marking areas, and a large amount of data to be encoded, the art proposed the use of matrix codes, especially the DataMatrix code, which reduces the required marking element size, precision and area, as well as contrast so that markings are able to be directly made on parts with, for example, steel or aluminum surfaces, and also proposed the use of imaging readers which use solid-state arrays similar to those used in digital cameras to capture an image of the marking. A microprocessor is used to analyze and decode the captured image of the matrix code.
Yet, the use of imaging readers, especially handheld readers, for reading marked workpieces has proven to be challenging. Contrast is still often less than desirable. Ambient lighting conditions are variable. Illumination from on-board illuminators is directed at variable angles. Reflections from ambient light sources and illuminators often appear in the field of view of the reader. Unlike machine-readable codes printed in one color (for example, black) on paper of another color (for example, white), DPM codes are typically difficult for a human operator to even find on the workpieces, which often have complicated shapes to further complicate finding the DPM code and aiming the reader directly at the DPM code for reading.